wifi: at76c50x: prefer struct_size over open coded arithmetic

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "cmd_buf" variable is a pointer to "struct at76_command" and
this structure ends in a flexible array:

struct at76_command {
	[...]
	u8 data[];
} __packed;

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + count" in the
kmalloc() function.

Also, declare a new variable (total_size) since the return value of the
struct_size() helper is used several times.

At the same time, prepare for the coming implementation by GCC and Clang
of the __counted_by attribute. Flexible array members annotated with
__counted_by can have their accesses bounds-checked at run-time via
CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
strcpy/memcpy-family functions). In this case, it is important to note
that the attribute used is "__counted_by_le" since the counter type is
"__le16".

This way, the code is more readable and safer.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160

 [2]
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/AS8PR02MB7237578654CEDDFE5F8C17BA8BFE2@AS8PR02MB7237.eurprd02.prod.outlook.com